"can snorting codeine be fatal"
[TLDR: Yes, potentially even more fatal than oral consumption]
"but codeine is metabolised in the liver. so snorting it would lead to less morphine and lower risk of respiratory depression"
No, that's not accurate—snorting codeine does bypass the liver's first-pass metabolism (which occurs after oral ingestion via the gastrointestinal tract and portal vein), but this actually leads to
higher overall bioavailability of codeine in the bloodstream compared to oral administration. Codeine is a prodrug with relatively weak intrinsic opioid activity (about 1/10th the potency of morphine), and its primary effects come from hepatic metabolism via the CYP2D6 enzyme to its active metabolite, morphine (typically 5-10% of the dose). However, the pharmacokinetics differ by route in ways that don't reduce risk:
- Oral route: Codeine undergoes partial first-pass metabolism in the liver, where some is converted to morphine before entering systemic circulation. Oral bioavailability is around 40-60%, meaning some codeine is lost to other metabolic pathways (e.g., to norcodeine or glucuronides) during this pass. This results in a slower rise in blood levels, with peak concentrations after about 1 hour.
- Intranasal route (snorting): Absorption occurs directly through the nasal mucosa into systemic circulation, bypassing first-pass and increasing codeine's bioavailability to nearly 100%. This means more unchanged codeine reaches the bloodstream quickly (peak in 10-20 minutes, similar to other intranasal opioids like heroin or hydromorphone). Once in circulation, the higher codeine levels are still metabolized by the liver over time via CYP2D6 to morphine. Because less codeine is "lost" upfront to non-morphine pathways, the total morphine produced can be comparable or even higher overall, though with faster kinetics due to the rapid codeine spike.
Codeine's oral-to-parenteral (e.g., injection or snorting equivalent) potency ratio is high (around 60%), unlike stronger opioids like morphine (where oral is much weaker due to extensive first-pass). This is because codeine experiences less overall first-pass inactivation, but bypassing it still amplifies peak exposure.
Respiratory depression (slowed or stopped breathing) is primarily driven by morphine's action on mu-opioid receptors in the brainstem, though codeine contributes weakly. Snorting doesn't lower this risk—it increases it due to:
- Faster onset and higher peaks: Rapid absorption leads to quicker, more intense opioid effects, including sedation and breathing suppression, raising the chance of acute overdose before the body can adjust.
- Dose equivalence issues: People often snort the same amount as an oral dose, but with higher bioavailability, this effectively delivers more drug systemically, potentially leading to excessive morphine buildup.
- Genetic factors: In ultra-rapid CYP2D6 metabolizers (5-10% of Caucasians), conversion to morphine is faster and higher regardless of route, amplifying risks. Poor metabolizers get little effect from either route.
- Compounding factors: Mixing with depressants (e.g., alcohol, benzodiazepines) or having conditions like sleep apnea heightens respiratory risks more with rapid routes like snorting.
Fatal doses of codeine are typically 500-1000 mg, but tolerance, mixtures, and route affect this. Snorting crushed tablets also introduces risks from fillers/irritants (e.g., acetaminophen in combos like Tylenol with codeine), which can cause nasal/lung damage or toxicity.
In summary, while snorting avoids initial first-pass conversion, it results in higher and faster codeine exposure, which doesn't reduce (and may increase) total morphine levels or the risk of respiratory depression compared to oral use at equivalent doses.